The widespread prokaryotic immune systems, in particular restriction–modification, CRISPR–Cas and defensive toxin–antitoxin systems, are absent in eukaryotes, whereas relatively rare ones, such as Argonautes and antiviral STAND NTPases, became central to eukaryotic innate immunity. In this Comment, we hypothesize that the cause of this paradox is the drastic reduction of horizontal gene transfer rate in eukaryotes.
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Acknowledgements
E.V.K. is supported by the Intramural Research Program of the National Institutes of Health of the USA. A.B. is supported by an ERC Starting Grant (PECAN 101040529).
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Koonin, E.V., Bernheim, A. The paradox of immune systems conservation between prokaryotes and eukaryotes. Nat Rev Microbiol (2026). https://doi.org/10.1038/s41579-026-01284-0
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DOI: https://doi.org/10.1038/s41579-026-01284-0
